Income Taxes - Deferred Income Tax (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets, Net [Abstract]
Net operating loss carryforwards	$ 61,049	$ 75,036
Research and development tax credits	3,731	2,131
Reserves and accruals	1,168	1,791
Other	6,611	4,594
Total deferred tax assets	72,559	83,552
Valuation allowance	(72,559)	(83,552)
Net deferred tax assets	$ 0	$ 0